Display screen or portion thereof with a graphical user interface



The FIGURE is a front view of a display screen or portion thereof with a graphical user interface showing the new design with environment in broken lines.

The outermost broken line illustrates the display screen or portion thereof and the broken line showing of text, numbers, data fields, and broken line depictions of line graphs illustrate portions of the graphical user interface. None of the broken lines form part of the claimed design. 

CLAIM The ornamental design for a display screen or portion thereof with a graphical user interface showing the new design, as shown and described. 